As is known, a single radar has a theoretical maximum signal to noise ratio (SNR) for a given size target at given range. The signal to noise ratio directly affects the ability of the radar to detect, track, and identify the target. The maximum signal to noise ratio is determined by a variety of radar characteristics, including but not limited to, the size of the radar antenna, the radar transmit power, and the radar receiver noise level. Each of these radar characteristics is substantially fixed for any particular radar. Therefore, in order to improve detection, tracking, or target identification performance, it has generally been necessary to design a new radar having new radar characteristics. Redesign of a new radar is a very long and expensive process and makes no use of existing radars.
Alternatively, it is possible to process together received signals from a plurality of radars (e.g., existing radars), each radar having a separate antenna, in order to increase the sensitivity of the combined system (i.e., to increase SNR for a given size target at a given range), and therefore, to increase detection, tracking, or target identification performance.
In order to process together the received signals, i.e., the target echoes from the plurality of radars, it is advantageous that the target echoes received by each of the plurality of radars be combined together coherently with the no relative phase difference. In this way, the improvement in radar sensitivity provided by the plurality of radars, compared with any one radar, is improved by the greatest amount.
One of ordinary skill in the art will understand that knowledge of the relative position of the radar antenna of each one of the plurality of radars to within a small fraction of a wavelength can allow phase corrections having sufficient accuracy to allow coherent in-phase processing. However, it is generally not sufficient that the position of the radar antennas merely be mechanically measured, since the distance between the radar antennas can be quite large compared to a wavelength, resulting in measurement inaccuracy. Furthermore, knowing only the mechanical separation between the radars is not sufficient since it does not account for internal electrical differences in time delay or phase shift between radars. Measurement of mechanical separation also does not account for a change in these electrical parameters with time due to temperature changes, aging, parts replacement, etc.
It would be useful to provide the above-described plurality of radars as mobile radars. However, mobile radars are subject to changes in relative position much greater than a wavelength, and therefore, calibration of relative position of mobile radar antennas would have to be performed each time the mobile radars are moved.
One of ordinary skill in the art will understand that calibrating the relative positions of a plurality of radar arrays is difficult and subject to increasing errors as the separation of the plurality of radar antennas increases. It will also be understood that such calibration is a separate process, requiring substantial time apart from actual operation of the radars.
When transmitting with a plurality of radars, for example, two radars, toward a target, each radar receives a monostatic target echo corresponding to its transmitted radar signal, and a bistatic target echo corresponding to a transmitted radar signal from the other radar. One of ordinary skill in the art will recognize that, if both the monostatic and the bistatic target echoes are received by each radar and processed together coherently, additional system sensitivity can be achieved beyond that achieved if only the monostatic target echoes are processed. However, the monostatic and bistatic target echoes received by each radar generally appear at different ranges (time delays) and with different phases, and therefore, do not generally sum coherently with no phase difference.